Modern Systems Analysis
and Design
Fifth Edition
Jeffrey A. Hoffer
Joey F. George
Joseph S. Valacich
Chapter 7
Structuring System Process
Requirements
Learning Objectives




Understand the logical modeling of processes by
studying examples of data flow diagrams
(DFDs).
Draw data flow diagrams following specific rules
and guidelines that lead to accurate and wellstructured process models.
Decompose data flow diagrams into lower-level
diagrams.
Balance higher-level and lower-level data flow
diagrams.
Chapter 7
© 2008 by Prentice Hall
2
Learning Objectives (Cont.)



Explain the differences among four types of
DFDs: current physical, current logical, new
physical, and new logical.
Use data flow diagrams as a tool to support the
analysis of information systems.
Discuss process modeling for electronic
commerce applications.
Chapter 7
© 2008 by Prentice Hall
3
Process Modeling
Chapter 7
© 2008 by Prentice Hall
4
Process Modeling (Cont.)

Graphically represent the processes that
capture, manipulate, store, and distribute
data between a system and its environment
and among system components.
A common form of a process model is a
Data Flow Diagram: A picture of the
movement of data between external entities and
the processes and data stores within a system
Chapter 7
© 2008 by Prentice Hall
5
Process Modeling (Cont.)
 Utilize
information gathered during
requirements determination.
 Processes and data structures are
modeled.
Chapter 7
© 2008 by Prentice Hall
6
Deliverables and Outcomes
 Context
Scope
 DFDs
data flow diagram (DFD)
of system.
of current physical system
Adequate
 DFDs
detail only.
of current logical system
Enables
analysts to understand current
system.
Chapter 7
© 2008 by Prentice Hall
7
Deliverables and Outcomes (Cont.)
 DFDs
of new logical system
Technology
independent.
Show data flows, structure, and
functional requirements of new system.
 Thorough
description of each DFD
component
Chapter 7
© 2008 by Prentice Hall
8
Data Flow Diagramming
Mechanics
 Represent
both physical and logical
information systems.
 Only four symbols are used.
Chapter 7
© 2008 by Prentice Hall
9
Definitions and Symbols
Chapter 7
© 2008 by Prentice Hall
10
Data Flow Diagramming
Mechanics (Cont.)
 Useful
for depicting purely logical
information flows.
 DFDs that detail physical systems
differ from system flowcharts which
depict details of physical computing
equipment.
Chapter 7
© 2008 by Prentice Hall
11
Definitions and Symbols (Cont.)
 Process: is the work or actions performed on
data so that they are transformed, stored or
distributed (inside the system), ie, generate
paycheck, calculate overtime, compute GPA
 Data store: is data at rest (inside the
system), ie, a file folder about customers or
students or orders, etc.
Chapter 7
© 2008 by Prentice Hall
12
Definitions and Symbols (Cont.)
 Source/sink: external entity that is origin or
destination of data (outside the system), ie,
customer, bank, inventory control system
 Data flow: arrows depicting movement of
data, ie, customer order, sales receipt
Chapter 7
© 2008 by Prentice Hall
13
Fig. 7.3 (a) and (b)
a: improper use b: proper use
14
Developing DFDs

Context diagram is an overview (highest
level) of an organizational system that shows:
 the
system boundaries;
 external entities that interact with the system;
 Major information flows between the entities and the
system.

Note: only one process symbol, and no data
stores (inside the process) shown.
Chapter 7
© 2008 by Prentice Hall
15
Context Diagram
Chapter 7
© 2008 by Prentice Hall
16
Developing DFDs (Cont.)

Level-0 diagram is a data flow diagram
that represents a system’s major
processes, data flows, and data stores at
a high level of detail.
 Processes
are labeled 1.0, 2.0, etc. These will
be decomposed into more primitive (lowerlevel) DFDs.
Chapter 7
© 2008 by Prentice Hall
17
Level-0
Diagram
Chapter 7
© 2008 by Prentice Hall
18
Data Flow Diagramming Rules

There are two DFD guidelines that apply:
 The
inputs to a process are different from the
outputs of that process.

Processes purpose is to transform inputs into
outputs.
 Objects

Chapter 7
on a DFD have unique names.
Every process has a unique name.
© 2008 by Prentice Hall
19
Table 7.2
Data Flow
Diagramming
Rules (Cont.)
Chapter 7
© 2008 by Prentice Hall
20
Decomposition of DFDs

Functional decomposition is an iterative
process of breaking a system description
down into finer and finer detail.
 Creates
a set of charts in which one process
on a given chart is explained in greater detail
on another chart.
 Continues until no subprocess can logically
be broken down any further.
Chapter 7
© 2008 by Prentice Hall
21
Decomposition of DFDs (Cont.)
Primitive DFD is the lowest level of a DFD.
 Level-1 diagram results from
decomposition of Level-0 diagram.
 Level-n diagram is a DFD diagram that is
the result of a n nested decompositions
from a process on a level-0 diagram.

Chapter 7
© 2008 by Prentice Hall
22
Fig 7.7 Level-1 diagram for Process 1.0
23
Level-1 DFD
Level-1 DFD shows
the sub-processes
of one of the
processes in the
Level-0 DFD.
This is a Level-1
DFD for Process
4.0.
Processes are labeled 4.1, 4.2, etc. These can be further
decomposed in more primitive (lower-level) DFDs if necessary.
Chapter 7
© 2008 by Prentice Hall
24
Level-n DFD
Level-n DFD shows
the sub-processes
of one of the
processes in the
Level n-1 DFD.
This is a Level-2
DFD for Process
4.3.
Processes are labeled 4.3.1, 4.3.2, etc. If this is the
lowest level of the hierarchy, it is called a primitive DFD.
Chapter 7
© 2008 by Prentice Hall
25
Balancing DFDs
 Conservation
Principle: conserve
inputs and outputs to a process at the next
level of decomposition.
 Balancing: conservation of inputs and
outputs to a data flow diagram process
when that process is decomposed to a
lower level.
Chapter 7
© 2008 by Prentice Hall
26
Balancing DFDs (Cont.)
 Balanced
means:
Number
of inputs to lower level DFD
equals number of inputs to associated
process of higher-level DFD
Number of outputs to lower level DFD
equals number of outputs to associated
process of higher-level DFD
Chapter 7
© 2008 by Prentice Hall
27
Balancing DFDs (Cont.)
This is
unbalanced
because the
process of the
context diagram
has only one
input but the
Level-0 diagram
has two inputs.
1 input
1 output
2 inputs
1 output
Chapter 7
© 2008 by Prentice Hall
28
Balancing DFDs (Cont.)
 Data
flow splitting is when a
composite data flow at a higher level is
split and different parts go to different
processes in the lower level DFD.
 The DFD remains balanced because the
same data is involved, but split into two
parts.
Chapter 7
© 2008 by Prentice Hall
29
Balancing DFDs (Cont.)
Chapter 7
© 2008 by Prentice Hall
30
Balancing DFDs: More DFD Rules
Chapter 7
© 2008 by Prentice Hall
31
Four Different Types of DFDs

Current Physical
 Process
labels identify technology (people or
systems) used to process the data.
 Data flows and data stores identify actual name of the
physical media.

Current Logical
 Physical
aspects of system are removed as much as
possible.
 Current system is reduced to data and processes that
transform them.
Chapter 7
© 2008 by Prentice Hall
32
Four Different Types of DFDs
(Cont.)

New Logical
 Includes
additional functions.
 Obsolete functions are removed.
 Inefficient data flows are reorganized.

New Physical
 Represents
the physical implementation of
the new system.
Chapter 7
© 2008 by Prentice Hall
33
Fig. 7-13 Current physical inventory
control system
34
Level-0 current and new logical
inventory control systems
35
Guidelines for Drawing DFDs
(Cont.)

Completeness
 DFD
must include all components necessary
for system.
 Each component must be fully described in
the project dictionary or CASE repository.

Consistency
 The
extent to which information contained on
one level of a set of nested DFDs is also
included on other levels.
Chapter 7
© 2008 by Prentice Hall
36
Guidelines for Drawing DFDs
(Cont.)

Timing
 Time
is not represented well on DFDs.
 Best to draw DFDs as if the system has never
started and will never stop.

Iterative Development
 Analyst
should expect to redraw diagram
several times before reaching the closest
approximation to the system being modeled.
Chapter 7
© 2008 by Prentice Hall
37
Guidelines for Drawing DFDs
(Cont.)

Primitive DFDs
 Lowest
logical level of decomposition.
 Decision has to be made when to stop
decomposition.
Chapter 7
© 2008 by Prentice Hall
38
Guidelines for Drawing DFDs
(Cont.)
 Rules
for stopping decomposition
When
each process has been reduced
to a single decision, calculation or
database operation.
When each data store represents data
about a single entity.
Chapter 7
© 2008 by Prentice Hall
39
Guidelines for Drawing DFDs
(Cont.)
When
the system user does not care to
see any more detail.
When every data flow does not need to
be split further to show that data are
handled in various ways.
Chapter 7
© 2008 by Prentice Hall
40
Guidelines for Drawing DFDs
(Cont.)
When
you believe that you have shown
each business form or transaction,
online display and report as a single
data flow.
When you believe that there is a
separate process for each choice on all
lowest-level menu options.
Chapter 7
© 2008 by Prentice Hall
41
Using DFDs as Analysis Tools
Gap Analysis is the process of
discovering discrepancies between two or
more sets of data flow diagrams or
discrepancies within a single DFD.
 Inefficiencies in a system can often be
identified through DFDs.

Chapter 7
© 2008 by Prentice Hall
42
Using DFDs in BPR
Figure 7-17 IBM Credit Corporation’s primary work process before BPR
Chapter 7
© 2008 by Prentice Hall
43
Using DFDs in BPR (Cont.)
Figure 7-18 IBM Credit Corporation’s primary work process after BPR
Chapter 7
© 2008 by Prentice Hall
44
Electronic Commerce Application: Process
Modeling using Data Flow Diagrams
Process modeling for Pine Valley
Furniture’s Webstore
 Completed JAD session.
 Began translating the Webstore system
structure into data flow diagrams.

 Identified
Chapter 7
six high-level processes.
© 2008 by Prentice Hall
45
Electronic Commerce Application: Process
Modeling using Data Flow Diagrams (Cont.)
Chapter 7
© 2008 by Prentice Hall
46
Electronic Commerce Application: Process Modeling using
Data Flow Diagrams
Figure 7-19 Level-0 data flow diagram for the WebStore
Chapter 7
© 2008 by Prentice Hall
47
Summary

In this chapter you learned how to:
 Understand
logical process modeling via
data flow diagrams (DFDs).
 Draw data flow diagrams of well structured
process models.
 Decompose data flow diagrams into lowerlevel diagrams.
Chapter 7
© 2008 by Prentice Hall
48
Summary (Cont.)
 Balance
high-level and low-level data flow
diagrams.
 Explain differences between current
physical, current logical, new physical, and
new logical data flow diagrams.
 Use data flow diagrams for analyzing
information systems.
Chapter 7
© 2008 by Prentice Hall
49